The nucleus of the atom

Atoms have a nucleus, which is built up of nucleones (nuclear particles, the most important ones: protons and neutrons.
The nucleus has its position in the middle of the atom.

The nucleon (one nuclear particle) has a mass of 1 a.m.u. (more about this later). All nucleons, nuclear particles toghether determine the atomic mass.
The presence of protons provides the nucleus with a positive charge. Where neutrons are neutral, every atom nucleus must have a postive charge. The (positive) nucleus attrackts the (negative) elektrons.

Changes in the nucleus are called: 'nuclear reactions'.